Apparatus for desalinating salt water by membrane distillation

ABSTRACT

An apparatus for desalinating sea water, comprising a distillation unit which includes a porous hydrophobic membrane which is pervious to steam but impervious to water, and a condensation surface arranged at a distance from the membrane, the distillation arrangement also including a first unit for heating and conducting the water to be distilled on one side of the membrane, and a second unit arranged to conduct a liquid which is colder than the water, on the side of the condensation surface remote from the membrane, distilled water being collected and drawn-off between the membrane and the condensation surface. 
     The apparatus is characterized in that the first unit incorporates a closed pipe loop (11&#39;,12,13) which is connected in series with the warm side (10) of the distillation unit (1); in that the second unit incorporates a closed pipe loop (14,15,16) which is connected in series with the cold side (11) of the distillation unit (1); in that a drainage valve (17) is provided for drawing-off water from the pipe loop (11,12,13), of the first unit; in that there is provided a water filling valve (20) and a pipe line (23) for drawing water from the pipe loop (14,15,16) of the second unit; in that the pipe loops of respective first and second units are connected together by means of a connecting line (25); and in that a heat exchanger (50) is connected to the pipe loop (11,12,13) of the first unit for supplying energy to the water present in the pipe loop of the first unit.

The present invention relates to an apparatus for desalinating saltwater by membrane distillation.

Apparatus utilizing membrane distillation are known to the art, interalia from the Swedish patent specification No. 419 699. The Swedishpatent specification describes an apparatus for membrane distillationcomprising a plurality of first cassettes for conducting warm salt waterand a plurality of second cassettes for conducting a cold liquid, suchas cold sea water. Located between each pair of mutually adjacentcassettes is a separation cassette which separates one of the firstcassettes from one of the second cassettes.

The separation cassette according to the aforesaid patent specificationincludes a hydrophobic porous membrane which constitutes one sidesurface of the cassette, and a thin plastics film which constitutes theother side surface of the cassette, the membrane being spaced from theplastic film so as to form an air gap between the membrane and the film.Warm salt water flows towards the membrane from the side thereof remotefrom the air gap and enters one of the first cassettes, whereas coldwater flows into one of the second cassettes on the side of the plasticfilm remote from said air gap.

Distillation is effected in a manner such that the hot or warm saltwater gives off water vapor which passes through the pores of themembrane and through said air gap, whereafter the water vapor condenseson the cold plastic film. The pores should be of a size which preventsliquid water from passing therethrough.

Such a distillation process affords a number of advantages; for examplethe process takes place at a temperature beneath the boiling point ofwater and approximately at atmospheric pressure, which enables anon-corrosive construction material to be used. In addition, such aprocess is relatively highly productive in relation to the size of thecassettes and the numbers thereof.

However, the process is encumbered with one serious disadvantage. Thisdisadvantage is based on the fact that large quantities of energy aretransported from the hot salt water on the evaporation side to the coldwater on the condensation side for each product water-unit desalinated.

This energy transport results in a lowering of the temperature of thehot water flow and a corresponding increase in temperature of the coldwater flow. Lowering of the temperature in the hot water flow and alsothe decrease in the temperature gap between the hot and the cold flowdecreases the production of fresh water. In order to maintain anacceptable level of productivity, it is necessary to restrict thesechanges in temperature.

This can only be achieved by increasing the flows of heated salt waterand cold water respectively through the desalinating unit.

This means that large quantities of salt water must be heated. Inaddition, large quantities of cooling water must be pumped through thedistillation unit.

In order for such a process to function satisfactorily the watersupplied to the process must be pre-treated by filtration and chemicaltreatment. Thus, the use of large quantities of water necessitatesexpensive heating and expensive treatment processes.

The present invention solves this problem, so that solely minorquantities of cold sea water supplied to the system need be heated andminor quantities of cold salt water need be supplied for coolingpurposes, at the same time as a significant part of the energytransferred from the hot salt water to the cold salt water is recovered.

The invention also enables the process to be readily controlled.

Thus, the present invention relates to an apparatus for desalinatingsalt water, comprising a distillation unit which includes a porous,hydrophobic membrane which is pervious to steam or water vapor but isimpervious to water, and a condensation surface arranged at a distancefrom the membrane, said distillation unit further comprising a firstunit for heating water to be distilled and conducting said water on oneside of the membrane, and a second unit arranged to conduct a liquid,which is colder than said water, on the side of the condensation surfaceremote from the membrane, distilled water being collected and drawn offbetween said membrane and said condensation surface, the invention beingcharacterized in that said first unit includes a closed pipe loop whichis connected in series with the warm side of the distillation unit; inthat said second unit includes a closed pipe loop connected in serieswith the cold side of the distillation unit; in that there is provided adrainage valve for drawing off water from the loop of the first unit; inthat there is also provided a water-filling valve and a pipeline fordrawing water from the loop of the second unit; and in that the loops ofthe first and the second units are connected together by means of aconnecting line; and in that a heat exchanger or the like is connectedto the loop of the first unit for supplying energy to the water presentin said loop.

The invention will now be described in more detail with reference to anexemplifying embodiment thereof illustrated in the accompanying drawing,the single FIGURE of which is a schematic flow diagram of an apparatusaccording to the invention.

FIG. 1 illustrates schematically and in cross-section a distillationunit 1 which includes the aforesaid cassettes and comprises a poroushydrophobic membrane 2 which is permeable to steam or water vapor butimpermeable to liquid, and further comprises a condensation surface inthe form of a plastic film 3 located at a distance from the membrane.Located between the membrane 2 and the plastic film 3 is an air gap 4which incorporates a channel or the like for communication with theatmosphere as illustrated by the arrow 5. Arranged at the lower end ofthe air gap 4 is a channel 6 or the like for conducting away freshwater-condensate from the air gap, as illustrated by the arrow 7. Themembrane 2 and the plastic film 3 may be suspended from any suitableconstruction.

Arranged on one side of the unit 1 is a first chamber 10 for conductingthe liquid to be distilled, as illustrated by the arrow 8, so that theliquid covers the side of the membrane remote from the air gap.

Arranged on the other side of the unit 1 is a second chamber 11 forconducting a liquid which is colder than the liquid to be distilled, asillustrated by the arrow 9, so that the colder liquid covers the side ofthe plastic film remote from the air gap.

According to the invention the first unit incorporates a closed firstpipe loop 11', 12, 13 which is connected in series to the first chamber10 of the distillation unit. The second unit also incorporates a closed,second pipe loop 14, 15, 16, which is connected in series with thesecond chamber 11 of the distillation unit.

The illustrated apparatus also includes a drainage valve 17 for drawingwater from the pipe loop 11', 12, 13 of the first unit, in the directionshown by the arrows 19. A pipe 18 extends from the closed loop 11', 12,13 to a drain, via the drainage valve 17.

A filling valve 20 for filling the loop 14, 15, 16 of the second unitwith water is mounted on a pipe 21 intended for supplying sea water atnormal sea-water temperature in the direction of the arrow 22, the pipe21 being connected to the pipe loop 14, 15, 16 of the second unit.

Also extending from the pipe loop 14, 15, 16 of the second unit is apipe 23 for drawing water from the pipe loop of the second unit in thedirection shown by the arrow 24.

The pipe loops of the two units are connected together at a locationthereon by means of a connecting line 25. When the apparatus isoperating, water is transferred intermittently or continuously throughthe connecting line 25 from the second unit to the first unit, asillustrated by the broken arrow 26. The arrows 8, 27 illustrated theflow direction in the first pipe line 11', 12, 13, whereas the arrows 9,28 illustrate the flow direction in the second pipe loop 14, 15, 16.

Each pipe loop incorporates a respective pump 29, 30 for advancing thewater through respective loops. A pump 31 is provided for pumping seawater into the second pipe loop, via the pipe 21.

The distilled water produced is carried away through a pipe 32, to acollecting vessel 33 or to a consumer.

Each distillation apparatus normally comprises a very large number ofdistillation units 1 connected together in parallel, although only onesuch unit has been shown in FIG. 1 for the sake of clarity.

As will be understood, the volumetric capacity of the collecting vessel33 in relation to the distillation unit 1 is much greater than thatillustrated in the FIGURE.

In the FIG. 1 the reference 34 identifies the surface of the sea,although it will be understood that the apparatus can be placed at someother location other than in the immediate proximity of the sea water.

According to the invention the drainage valve 17 is controlled by meansof a control device 35, via an electric conductor 36. The valve 17 issuitably a magnetic valve, i.e. a valve controlled by an electromagneticdevice 37. The control device 35 is arranged to control the valve 17 inresponse to a signal emanating from a transducer 38 in the form of asalinometer of a kind known per se. The signal is relayed on an electricconductor 57. This signal constitutes a real-value signal. The controldevice 35 incorporates a setting means 39, for example in the form of apotentiometer, for setting a control value, or a control value interval.

When the water present in the first loop 11', 12, 13 is distilled, steamis transported through the membrane 2, causing the salt content in theloop to rise. In conjunction herewith the control device 35 receives asignal from the salinometer 38, indicating an increase in salt content.The control device 35 is arranged herewith to cause the drainage valve17 to open the connection between the first pipe loop and the freeatmosphere, in FIG. 1 illustrated by the sea-water surface 34.Introduction of further water is effected from the second pipe loop 14,15, 16, via the aforementioned connecting line 25, synchronously withdraining water through the pipe-line 18. Since the salt content of thewater in the second pipe loop is lower than the elevated salt content ofthe first pipe loop the salt content of the first pipe loop will fallwhen introducing further water thereinto. The control device 35 isarranged to close the valve 17 when the salt content has fallen to apredetermined level set through the setting means.

Thus, the first loop is filled solely with water from the second pipeloop, through the single connecting line or pipe 25 between the twoloops.

The apparatus as a whole is replenished with water solely through theaforementioned filling valve 20, i.e. to the second pipe loop 14, 15,16. The filling valve 20 is also controlled by an electromagnetic device40, i.e. a magnetic valve. The filling valve 20 is controlled by meansof a control device 35, which is preferably the same device as thatwhich controls the drainage valve 17, but which may also be a separatecontrol device.

The control device controls the filling valve via an electric conductor61.

The control device 35 is arranged to control the introduction of waterto the second pipe loop 14, 15, 16 in response to a signal transmittedfrom a temperature sensor 42 over an electric conductor 41, in a mannerto maintain a pre-determined temperature in the pipe loop of the secondunit.

This signal constitutes a real-value signal. The control device 35incorporates a setting means 43, for example in the form of apotentiometer, for setting a control value or a control value interval.

In order to obtain effective distillation, it is necessary for atemperature difference to prevail between the two pipe loops ofrespective units. By filling the apparatus with water through thefilling valve 20, either intermittently or continuously, at the sametime as the amount of water introduced is substantially less than theflow in the pipe loop 14, 15, 16, a sufficiently low temperature ismaintained in the second unit without needing to supply the second unitcontinuously with large quantities of water. Thus, far less water needbe filtered and treated with chemicals than when the quantity of waterintroduced into the second unit is equal to the flow in the said secondunit. A filtering and chemical-treatment stage 44 is illustratedschematically in the FIGURE.

Since a pre-determined lower temperature is to be maintained in thesecond unit, however, it is necessary for the amount of water introducedvia the filling valve to exceed the amount of water transferred to thefirst pipe loop via the connecting pipe 25, unless a separate coolingdevice is incorporated in the pipe loop of the second unit. In view ofthis, the aforesaid pipe 23 is provided for the purpose of taking waterfrom the second pipe loop.

According to one preferred embodiment of the invention, the aforesaidpipe 23 extends via a vessel 45 provided with a spillway 46, from whicha pipe 47 extends into the sea or ocean 34.

In accordance with the invention, the volumetric capacity of the vessel45 is sufficient to replace the volume taken from the first unit at oneand the same point in time via the drainage valve 17. In this way it isnot necessary to synchronize drawing of water from the first unit withcorresponding filling of the second unit with water via the fillingvalve 20. According to one embodiment, the vessel 45 is provided with alevel sensor 48, which is arranged to send a first electric signal tothe control device 35 over a conductor 49 when the level of water in thevessel has fallen to a pre-determined value. When this pre-determinedlevel is reached, the control device 35 causes the filling valve 20 toopen, and holds the valve in its open mode until the level sensor 48sends a second signal to the control device 35, this second signalsignifying that the level has been restored to a pre-determined level.

According to a further, preferred alternative embodiment, however, thecontrol device 35 is adapted to open the filling valve 20 at the sametime as the aforesaid drainage valve 17 is opened, so that water isintroduced synchronously into the second unit when water is transferredto the first unit via the connecting pipe 25.

In order to obtain the requisite water temperature in the first unit,the water passes a heat exchanger connected to an energy source 51.

Although the energy source 51 may be any suitable source, it preferablycomprises a source of waste heat. For example, the heat source maycomprise the cooling water system of a diesel engine used to produceelectrical energy for driving the aforesaid pumps and control devices.In those countries which enjoy a warmer climate solar panels may beused, either separately or in combination with a waste-heat source. Whenthere is adequate access to electrical energy, the energy source 51 maycomprise a source of electrical energy.

One characteristic of a membrane distillation process according to theaforementioned Swedish patent is that the flow through the membrane 2greatly increases progressively with increasing temperature of the waterto be distilled. In this case, the cooling water in the second unit isassumed to be held at a substantially constant level.

When applied to the present invention, this means that the processbecomes self-regulating such that the process itself finds its ownworking level, due to the fact that the amount of water distilled in thefirst unit corresponds to the amount of energy supplied, wherewith acorresponding amount of pre-heated water is also supplied to the pipeloop of the first unit from the pipe loop of the second unit, via theconnecting pipe. The supply of water to the pipe loop of the second unitfor maintaining the aforesaid pre-determined temperature therein is alsodirectly dependent on the amount of water distilled.

By way of example it can be mentioned that when the water in the firstunit has a temperature of 80° C. and the water in the second unit ispermitted to have a temperature of 60° C., the relationship between theflows in the process is as follows:

Quantity of distilled water via the pipe 32; quantity of water drawnfrom the second unit via the pipe 23; quantity of water drawn from thefirst unit via the pipe 18 =1:10:2.

Those energy losses occurring in a process of this kind are primarilydue to heat transport through the membrane 2. Owing to the fact that aseparate cooling loop is provided, mainly the second pipe loop, asignificant part of these energy losses is recovered by heating thewater in the loop. Since this water is transferred to the first pipeloop, the first unit is therefore supplied with pre-heated water.

As will be clearly seen from the aforegoing, the present inventionsolves the aforesaid problems regarding the large quantities of waterneeding to be filtered, treated with chemicals and heated. In addition,a significant part of the energy transported through the membrane isrecovered. The apparatus also finds its own working level in dependenceon the energy supplied.

In order to further improve the efficiency of the apparatus and reducethe amount of water which need be supplied, lied, there is provided inaccordance with a preferred embodiment a heating arrangement comprisinga heat pump 52 of a known kind. The evaporator 53 of the heat pump 52 isconnected to the second pipe loop via a heat exchanger, and thecondenser 54 of said heat pump is connected to the first pipe loop via aheat exchanger. Thus, energy is transferred via the heat pump from thesecond unit to the first unit, as illustrated by the arrow 55.

According to a modified embodiment, the heat pump includes a furtherevaporator 56 connected, via a heat exchanger, to the pipe 6 throughwhich distilled water is removed from the apparatus. This enables partof the heat content of the distilled water to be recovered.

By using a heat pump in the aforedescribed manner, substantially allenergy lost from the distillation process is recovered.

The reference numerals 58, 59, 60 in the FIG. 1 identify one-way valveswhich permit water to flow in solely one direction, from the non-filledside to the filled side of the valve.

In the aforegoing, the invention has been described with reference toone embodiment thereof. It will be understood, however, thatmodifications can be made to the illustrated embodiment. For example,the illustrated components can be replaced with other components havingan equivalent function, and the schematically illustrated couplingdiagram of the pipe system illustrated in the FIGURE can be modifiedwithout departing from the concept of the invention, namely thearrangement of a first unit in which the warm water flows in a loop, thearrangement of a second unit in which the cold water flows in a loop,and the joining of the loops so that the first unit is filled by takingwater from the second unit.

Thus, the present invention is not restricted to the aforedescribed andillustrated embodiment, and modifications can be made within the scopeof the following claims.

We claim:
 1. An apparatus for desalinating salt water comprising: adistillation unit including a porous hydrophobic membrane which ispervious to steam but impervious to water; a condensation surfacelocated at a distance from the membrane; a first unit for heating andconducting the water to be distilled on one side of the membrane; asecond unit arranged to conduct water which is colder than the water tobe distilled on the side of the condensation surface; means fordrawing-off distilled water between said membrane and said condensationsurface; a closed pipe loop (11', 12, 13) which is connected in serieswith the warm side (10) of the distillation unit (1); a closed pipe loop(14, 15, 16) which is connected in series with the cold side (11) of thedistillation unit (1); a drainage valve (17) for drawing water from thepipe loop (11, 12, 13) in the warm side of the distillation unit afilling valve (20) for filling with water in the cold side of thedistillation unit; a pipe line (23) for drawing water from the pipe loop(14, 15, 16) of the second unit; a connecting line (25) connecting therespective pipe loops of the first and the second units; and a heatexchanger (50) connected to the pipe loop (11', 12, 13) of the firstunit for supplying energy to the water present in the pipe loop of thefirst unit.
 2. Apparatus according to claim wherein, the drainage valve(17) is controlled by means of a control device (35) arranged to controlthe withdrawal of water in response to a signal from a salinometer (38)incorporated in the pipe loop (11', 12, 13) of the first unit, so as tomaintain a pre-determined salt content in the pipe loop (11', 12, 13) ofthe first unit.
 3. Apparatus according to claim 2, wherein the fillingvalve (20) is controlled by means of a control device (35) arranged tocontrol a filling operation in response to a signal from a temperaturesensor (42) to maintain a pre-determined temperature in the pipe loop(14, 15, 16) of the second unit.
 4. Apparatus according to claim 3,wherein said control device (35) includes means such that filling valve(20) is opened at the same time as the drainage valve (17) is opened. 5.Apparatus according to claim 1, wherein the pipe line (23) of the secondunit is connected to a vessel (45) provided with a spillway (46). 6.Apparatus according to claim 1, wherein the first unit further includesheating means which comprises a heat pump (52), the evaporator (53) ofwhich is connected to the pipe loop (14, 15, 16) of the second unit, andthe condenser (54) of which is connected to the pipe loop (11', 12, 13)of the first unit.
 7. Apparatus according to claim 6, wherein the heatpump (52) includes a further evaporator (56) which is connected to apipe (6) for drawing-off distilled water.